1. Field of the Invention
The present invention relates to a terahertz spectrometry device and method and to a nonlinear optical crystal inspection device and method.
2. Description of the Related Art
Conventionally, there has been proposed a frequency sweep terahertz spectrum measurement device which, in the case of frequency calibrating the terahertz spectrum of a measurement sample, separates reference light from terahertz light applied to the sample while performing a frequency sweep and performs intensity correction of the terahertz light to be applied (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 2008-268162).
There has also been proposed an apparatus for investigating a sample, the apparatus comprising: an emitter for irradiating the sample with a beam of emitted electromagnetic radiation; and a detector for detecting the radiation reflected from the sample, wherein there is an optically nonlinear member which functions as both an active part of the emitter and an active part of the detector, said emitter and detector using the same part of the optically nonlinear member (e.g., see JP-A No. 2003-529760). In the apparatus of JP-A No. 2003-529760, targeting reflection measurement, the optical paths for the application and detection of the terahertz waves are made the same and a terahertz reflection mirror is installed on the application optical path, whereby terahertz waves are generated and detected and reference light is acquired.
Further, it is known that, in a semiconductor laser excitation laser device, an intensity of laser output light that is output from the laser device changes according to a wavelength or intensity of excitation light. In this regard, a semiconductor laser excitation laser device has been proposed, in which an intensity of laser output light is detected, and in accordance with the value thereof, a driving current and a temperature of a semiconductor laser for excitation is controlled so as to stabilize the intensity of the laser output light (e.g., see JP-A No. H05-029695).
Generally, in order to raise measurement sensitivity in spectral analysis, it is extremely important to improve the signal-to-noise ratio (S/N ratio)—that is, to increase signal strength and reduce noise. Further, in order to raise measurement reproducibility, it is important to perform intensity correction of the measurement light taking changes in the intensity of the light of the light source as a reference. This also applies to terahertz spectral analysis, and a high-intensity terahertz light source and intensity correction of the measurement light are essential.
However, the technology of JP-A No. 2008-268162 acquires the reference light for performing intensity correction of the terahertz measurement light by separating it from the terahertz light source. This means that the intensity of the terahertz waves applied to the measurement object—that is, the intensity of the light of the light source—drops, and there is the problem that the technology cannot realize a high-intensity terahertz light source, which is one factor for realizing high sensitivity and high reproducibility in spectral analysis.
Further, even in a case where the light of the light source is laser output light, a portion of the laser output light that has been output from the laser device is taken out in the technology of JP-A No. H05-029695 in order to stabilize the intensity of the laser output light, and the problem that the intensity of the light of the light source irradiated to the measurement object drops similarly exists even in the case where the light of the light source is laser output light. In particular, because terahertz waves by nature are weak in intensity, when the reference light is separated from the terahertz light source, the drop in the intensity of the terahertz light source ends up becoming pronounced.
Further, in a method that separates the reference light from the terahertz light source like in the technology of JP-A No. 2008-268162, two detectors for detecting the terahertz waves—a detector that detects the terahertz waves that have passed through or been reflected from the measurement object and a detector that detects the terahertz waves that have been separated as the reference light—become necessary, and there is also a problem in terms of cost because terahertz detectors are expensive.
Further, the technology of JP-A No. 2003-529760 detects the detection light and the reference light on the same optical path by inserting the terahertz reflection mirror, so the detector for detecting the terahertz waves can be configured by a single detector. However, the technology of JP-A No. 2003-529760 has the problem that it cannot perform intensity correction with good precision because the timings when the detection light and the reference light are acquired differ.